This invention relates to a timer, and more particularly, to a characteristic timer for a protective relay utilized for the protection of a portion of a high voltage transmission line.
Protective relays are commonly employed to detect and respond to faults that may occur within a protected portion of a high voltage transmission line. The output stage of the protective relay frequently employs a characteristic timer having a specified activating time response to a coincidence-type developed signal generated by the protective relay. The characteristic timer typically controls the output of the protective device. If the duration of the coincidence signal equals or exceeds the specified setting or activation time of the characteristic timer the circuit breaker is activated and the protected portion is decoupled from the remainder of the high voltage transmission line.
The specified activation time is typically dependent upon the frequency of the alternating current conducted by the high voltage transmission line and the required protective characteristic shape. The protective relay may desire to generate an alarm signal when the phase coincidence between two sensed signals of the protective portion equals or exceeds 90 degrees. The 90 degree relationship is related to the frequency of the A.C. source. For example, for a 60 Hz A.C. source one cycle of 360 degrees has a duration of 1/60 Hz. or 16.66 msec, so that a 90.degree. phase coincidence has a duration of 16.64/4 msec or 4.16 msec. Similar manipulations for a 50 Hz signal reveals that a 90 degree relationship has a value of 5.0 msec. It is considered desirable that the characteristic timer of the protective relay have means for which its setting time and angular relationship may be easily altered so as to easily adapt the protective relay to various high voltage transmission lines conducting or transmitting various alternating current.
The characteristic timer typically located in the output stage of the protective relay provides the final response of the protective relay to an abnormal condition within the protected portion of the high voltage transmission line. The protective relay may employ a phase comparator method for determining an abnormal condition of the high voltage transmission and generating a representative signal which is applied to the characteristic timer. The phase comparator method may provide a representative output signal to the characteristic timer when phase coincidence between a polarizing signal V.sub.POL and an operate signal V.sub.OP exists. The pulse width of the waveshape of the phase coincidence output signal may be in the form of blocks spaced apart by "holes". It is desired that the activation time of the characteristic timer be primarily determined by the pulse width of the blocks and have means to compensate for the pulse width indicative of the holes.
One such timer is described in U.S Pat. No. 4,180,842 of M. F. Keeney, issued Dec. 25, 1979, assigned to the same assignee of the present invention and herein incorporated by reference. It is still considered desirable that the characteristic timer be provided with inherent or built-in error detection means so as to filter or discriminate against the "holes" that might be generated by the phase comparator circuit of the protective relay.
Furthermore, the characteristic timer being located in the output stage of the protective relay adds to the "dropout time" or "resetting time" of the protective relay. The dropout or resetting time determines, in part, the speed of response of the protective relay scheme. Accordingly, lowering the resetting time enhances the speed of response of the protective relay scheme. It is considered desirable to provide a characteristic timer having a relatively short resetting time so as to enhance the overall response to the protective scheme which includes other protective functions such as circuit breaker failure.
It is one object of the present invention to provide a characteristic timer for which its activation time is easily alterable so as to adapt the protective relay to various high voltage transmission lines transmitting various alternating current.
It is another object of the present invention to provide a characteristic timer having means to discriminate or ride over transients or "holes" that may appear in phase coincidence signal developed by a phase comparator method of a protective relay so as to provide an output response for a protective relay which is substantially free of error.
It is a further object of the present invention to provide a characteristic timer that contributes to a relatively short resetting time so as to enhance the speed of response to the protective relay scheme.
It is a still further object of the present invention to provide a characteristic timer comprised of digital devices.
These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.